1. Field of the Invention
The invention relates to a method of fabricating a suspended structure, and more particularly to a method using a gas injection process on a structural layer to prevent the structural layer from sticking to a substrate after removing a sacrificial layer.
2. Description of the Prior Art
Micro-electromechanical systems (MEMS) are modern technology, which coordinate electrical circuits and mechanics. MEMS have been applied to fabricate devices with both electrical circuits and mechanical elements, such as pressure sensors, accelerators, and micro-microphones. Compared to semiconductor devices, MEMS devices usually have a suspended structure, for instance, a diaphragm. While fabricating a MEMS device, a sacrificial layer is formed before a structural layer, which is going to be the structure of the device. Afterward, the pattern of the structural layer is defined, and then the sacrificial layer is removed to form a suspended structure.
In general, the material of the sacrificial layer is silica, and the material of the structural layer is silicon. There are two major methods of removing the sacrificial layer: dry etching and wet etching. Dry etching uses a gas-phase material, such as a fluorine-included gas, to remove the sacrificial layer and to form the suspended structure. However, dry etching costs much process time. Contrary to dry etching, wet etching uses a liquid, such as a hydrofluoric acid (HF) solution, to remove the sacrificial layer and to form the suspended structure. Wet etching costs less process time. Nevertheless, the suspended structure may collapse easily owing to the surface tension of the cleaning solution and stick to the substrate during the following cleaning process.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 illustrate a conventional method of removing the sacrificial layer by wet etching. As shown in FIG. 1, a substrate 10 is provided. The substrate 10 comprises a sacrificial layer 12 with a defined pattern, and a structural layer 14 on the front surface of the substrate 10. Afterward, an opening 16 is formed on the back surface of the substrate 10. As shown in FIG. 2, a wet etching process is performed to etch the sacrificial layer 12 via the opening 16 by an etching solution and to form a suspended structure 18. Then, a cleaning process is performed immediately. The suspended structure 18 is a fine structure and is able to shift vertically in a proper range. However, according to the conventional method of removing the sacrificial layer 12 by wet etching, the suspended structure 18 may collapse owing to the surface tension of the cleaning solution and stick to the substrate 10. These may result in device damage and affect the yield of fabrication seriously.